Method of purifying zinc concentrates



Patented July 28, 1942 ,METHQD F PURIFYING "CQNP A B "zINo Q I Carl 0, Anderson, Baxter springsl kans, Rudolph J.-Stengl, Bosiclar'e, Ill., bott, Tulsa, Okla., assignors to Mahoning'Mining CompamnYoungsto 3501 Delaware and. Frederick 0. Ab,

w n',' Ohio, a corporation 7 Serial No. 4:17, 116

I NoDrawingsApplication()ct0ber ,30,1941, j

t if m (c1. 75m

'I'hls"invention relates -to thepurification of zincsulfide; or 'sphalerite, and especially to a novel method of freeing zinesulfide flotation concentrates or the like from fluorine-bearing w Contaminants to thereby render the concentrates suitable for use in zinc smelting plants and their 'sulfur'co'ntent available .for the manufacture of 'sulfuricacid. w i

' -Fluorspar is objectionable in zinc concentrates because its fluorine contentis driven off during the roasting, sinterin'g, and perhaps in: the retorting (smelting), usually as hydrogen'fiuoride (HF),- and readily attacks-the smelter refractories and otherparts small amount is present damages the apparatus, while the .gases driven off during roasting and sintering cannot beused for --making-sulfuric acid-if theyscontain eve slight amounts of fluorine compounds. 7

- 'Another objection to the use of zinc concentrates containing fluorspar isth'at in many corn -munities the liberation of fluorine-bearinggases is prohibited by law and/or regulations off public even when only avery and so in time :seriously authorities and the smelters must therefore make some provision for eliminating any'fluorine content before discharging gaseous smelter wastes into the atmosphere. v Consequently; mostsmeltersJeven though not engaged in sulfuric acid manufacture, refuse to accept concentrates containing appreciable fluorspar, while those who: will accept 1 concentrates containing up to possibly 1% of it generally exact penalties in the form of lower prices'to the concentrate producers.

' -It hastherefore beenextremely difficult to suc- "cessfullycommercialize certain large deposits of -'fiuo'rine-bearing sphalerite, such, for example, as -those in-the Cave-In Rock district of Southern --Illinois, where ores relative rich in zinc sulfide? large quantities, since because of "occurin fairly "the'fluor'spar in the ore the zinc concentrates produced therefrom cannot be: entirelyifreed of 1 it 'by-flotati'on'and therehas been no;-commersphalerite-fluorsparores. I n A'further'objectisthe provision of a method lnventionwill be V tion of it proceeds.

- utilized totreatdir the said method ,has not entirely. solved the problem from a commercial standpoint. Q 1' a I s The presentinvention isaccordingly directed eliminationof i all perceptible traces of fluorine'from zinc concentrates and the like and is especially concerned with the provi- I sion of an industrially acceptable method ofpurito, the complete fication -torender-them suitable for; use in -zinc smelters and innocuous, to sulfuric acid manufacturing units associated therewith.

;It-is, consequently, a principal' o bject" of.tl'ie invention to provide a novel lng fluor ne-free zinc-sulfide from concentrates 1 are c ontaminated with fiuorspars-fv Another, object of-the invention is the. prov-isionof amethod for treating zinc sulfide concentrates '.to, eliminate contained fluorspar in 'a relatively short time and with a minimum of dabor and reagent expense -whereby fluorine-free zinc-sulfide competitive with that obtained from ores devoid'of; fluorspar can be-pr'oduced' from of; freeing zinc sulfide, concentrates from .fiuorineqbytreating them WithfilllfdliC, acid in the ,presence ofjan aluminum-bearing compound; j n

Other objects, purposes and advantages. vofqthe understood, by or be" apparent to those skilled in the art .as the following descrip- While the inventionr- -may beadyanta d sly ectly atfiotation; froth; containing fluorspar cont'aminated zinc-sulfide concentrates, it. will .be

contaminated zinc sulfide ore in relativelyef nely divided condition canv be treated with equal facihtyand, consequently, by theuse herein; of the we intend to include the latter however. der ved. Zinc. sulfide flota-tion froths usuallyv comefrom the flotation circuit "at a density of about 40% solids and; to condition themfor the treatment suchvfroths ,are des'i-i'ably --cially satisfactory way-of otherwise elimin'at- '--ingit.' a Weare familiar with the method-of removing fiuorsp'arfrom zine concentrates described and f-claimedinu's; Patent 2,137,600, issued Novemher 1221938, to F. C. Abbott 'et al-.; but'have' found a in practice thatrthis method; while efiectiveto' reduce the fluorspar "contentofsthe concentrates tosmall-fractions' of 1%,-is aslow. process and not fully effectiveito remove all, or'allvbut an imperceptible trace," of the fluorine unless prolonged for uneconomic periods of. time. though-capable of giving good resultsapart from :the que'stionsaof timeand operating convenience,

:Therefore, .-al-" thickened in 4a suitable vto ,a density of'about 55-65%; solids,- while ;if a

weight of water is added J'suitable consistency.

' ened froth or 'ing about 607 "which yields point of-speed'of reaction and other factors," and settling tankor the; like dry: ore is .used approximately two-thirds -ofits to produce a pulp of In accordance Withtheinvcntion, .tl'ie lthickpulp constituting a slurry contain- 'solidsis first mildly heated, 'pieierably to a' t'emperature of about 50-90 C.

optimum results from the'stnndafter,'or if preferred during, theinitial heating, about pounds understood any -,similarly' I "of concentrated" sulfuric" acid (661 B'.) per ton-of dry'concentratcsand apreing filtering operation. from the thickener carries;

determined quantity of a suitable aluminumbearing compound, such as alumina or bauxite, are added to and thoroughly mixed with the warm slurry. When refined alumina, aluminum chloride, aluminum sulfate or the like is used as the aluminum-bearing ingredient a slight excess over stoichiometrical equivalency to the fluorspar present is employed, bauxite we prefer a considerably greater excess,

running up to about 50 pounds per ton of dry concentrates when the latter contain not more than about 1% CaFz.

The mass of slurry, acid, and aluminum-bearing compound is kept in constant motion until the reaction of the several ingredients goes to completion, heat being supplied in any convenient wayto keep the temperature substantially constant. Thuswith the aid of a steam jacket on one conduit connecting a circulating pump with the reaction receptacle or tank we can readily provide both the heat and the agitation desired. J

Usuallythe reaction is complete after about one or two hours or less as indicated by the entire decomposition of the calcium fluoride and it is ordinarily unnecessary to continue it for longer than two hours. Whether or not it is completed at any time, however, can easily be ascertained by analyzing a washed sample of the solids to determine if it contains any perceptible fluorine; if it does not, the reaction is complete and the mass consequently ready for further treatment.

It is, therefore, removed from the reaction tank and, particularly if corrosion-resistant flltering means are not readily available, is preferably diluted with water and thickened by settling once or twice before subjecting it to the succeed- The overflow discharged off a large proportion of the excess acid and a substantial proportion of the fluorine content of the ore in solution and corrosion by these agents of apparatus used later in the method is thereby prevented. Excess acid remaining in the underfiow or thickened mass may then be neutralized with soda ash, calcium hydrate, or other ble of reacting with the residual acid to produce soluble or innocuous products only, and the neutralized slurry is next filtered, conveniently at the ordinary temperature at which it usually reaches the filter. The solid material or filter cake deposited on the filter is washed with water and then, if necessary, with a weak alkali solution to insure complete neutralization of all traces of acidic residue before it is removed from the filter and dried preparatory to delivery to smelters or other users as fluorine-free zinc concen-' trate.

The filtrate or waste liquor from the filter and the overflow from the thickener contain both aluminum andfluorine values and under some conditions it may be desirable to recover them,

but we have found this unessential to the economic practice of our method as the cost of operating itis fully met by the increased value of the purified zinc concentrates produced; the filtrate and thickener overflow will thus usually be retained only when they can be advantageously utilized in some way or treated to recover their contained values at a profit.

In the practice of ourmethod certain chemical reactions occur the precise nature of which is not fully understood arid they are not at present susceptible of being set forth with absolute certainbut when using raw or crude practical relatively harmless alkali capapresent, whether in and an excess of extent. This hypothesis a the fluorine content of the ore to soluble form,

making possible its removal in solution when either alumina (aluminum oxide, A1203) or its equivalent for chemical purposes, bauxite-(which consists essentially of aluminum oxide (A1203) and water) is used.

acara sHzso4+A1lo3=2A1F3+scaso -srao This reaction results in the production of a certain amount of calcium sulfate '(CaSO4)" and a little more of it may result from reaction of some of the excess sulfuric acid with the small amount of calcite (CaCOa) often present in the feed, and/or with calcium hydrate (Ca(OH)z) when it is used just prior to filtering for neutralizing any remaining residual acid. I

But calcium sulfate formed in this way is probably in the hydrated state as gypsum (CaSO4.2H2O) and its solubility is usually sufllcient and its occurrence is in such minor quantities that it does not become a significant impurity in the end product. That is, all the calcium fluorspar, in calcite, or in calcium hydrate, is theoretically converted to the slightly solubl calcium sulfate or gypsum, but most of it is removed in solution or in suspension in the large quantities of water comprised in the thickener overflow and in the filtrate and wash water; the minute amount of it which remains in the filter cake is consequently so small it can be disregarded.

It, is important to note, however, 'that the he fluorine content of the concentrates, which by customary flotation methods can be and usually is kept below about 0.49% (1.0% CaFz), and is desirably never permitted to exceed about 1.22% (2.5% CaFz) in concentrates intended for treatment by our method, is entirely converted to a soluble compound which is identified in the above equation as aluminum fluoride (AlFs). This material is so soluble that it is readilyremoved in solution during the course of the treatment and while it is formed in the presence of zinc sulfide sulfuric acid, such removal is not accompanied by loss of appreciable quantities of the zinc content of the concentrates.

It is our present belief, this may be due to'the fluorspar being first rapidly decomposed by the sulfuric acid and the fluorine ion thus released then reacts with the alumina, with the aluminum content of the bauxite, or with whateverother aluminum-bearing material isused, to produce the soluble aluminum fluoride (AlFz) rather than attacking the zinc sulfide directly or, after combining with hydrogen, while the calcium released from the fluorspar tends to neutralize in part the excess sulfuric acid so that neither ion can attack the zinc sulfide to any appreciable would largely account for the absence of material decomposition of the zinc sulfide and/or zinc losses which often occur to a distinctly significant degree in other methods involving the treatment of zinc sulfide in the presence of excess sulfuric acid, but it is possible some other conditions or reactions may the latter nor the fluorine we therefore supply a substantial excess of the acid over the stoichiometric equivalent of the fluorspar present to enable this reaction to occur without impeding complete removal of the fluorspar.

If any such reaction of bauxite with the sul furic acid takes place, it minimizes to a corresponding extent the residual acidity which might otherwise result from, the presence of the excess acid and thus not only possibly prevents it from attacking the zinc sulfide but also incidentally may facilitate the subsequent diluting and neutralizing steps, if they are employed, so we ordinarily supply more bauxite than the amount equivalent to the fluorspar present to allow for this condition.

Other reactions than the ones we have herein described also probably occur during the practice of our method but while we are able to describe the results, as above noted we, are not prepared to state with positive certainty precisely what such reactions may be or even that those to which we have referred invariably take place.

Accordingly, it may be considered our method contemplates the treatment of zinc sulfide concentrates or generally similar relatively finely divided zinciferous minerals containing residual fluorspar contamination which preferably does not exceed 2.5% (about 1.22% fluorine), and that its principal steps include digesting the material in the presence of a substantial quantity of anj aluminum-bearing compound and sulfuric acid in considerable excess over the stoichiometric equivalent of the fluorspar.

Inasmuch as this digesting step is attended by substantially all the essential chemical activity required to convert the fluorine to water soluble compounds, it will be evident that the subsequent steps, including diluting, in some instances followed by neutralization of excess acid and thickening, ultimate filtration, washing and.

drying of the filter cake, and disposal of the re: sidual liquors, wash solutions and other fluids practically are all mechanical'operations which can readily be carried out either as herein described or in some equivalent way, and further that the method may be practiced as either a batch or a continuous one as preferred.

It will nowbe apparent our invention permits the production from fluorspar-containing concentrates of fluorine-free zinc sulfide which is as acceptable to operators of zinc smelters and sulfuric acid plants as zinc sulfide produced from naturally fluorspar-free ores, and that the practice of the invention does not entail the use of extremely high temperatures, unduly prolonged reaction times, delicate balancing of reagents, or, save for the reaction apparatus itself, employment of acid proof equipment.

While the appended claims may refer in more or less specific terms to one or more of the several operations heretofore mentioned as incident to the practice of our method, it will be understood they may be changed and/or modified in their several details and that the relationships between the various steps comprised in the method may be adapted to suit the preferences or convenience of the operator without departing from the true spirit and scope of the invention as defined in the said claims.

Having thus described our invention, we claim and desire to protect by Letters Patent of the United States:

A method of freeing zinc sulfide concentrates of a fluorine content not in excess of about 1.25% present as calcium fluoride which comprises subjecting the concentrates at moderately elevated temperatures to the action of sulfuric acid in substantial excess of the StDiChi-v ometrical equivalent of the fluorine and in the presence of aluminum oxide in not less than the trates of a fluorine content not in excess of about 1.25% present as calcium fluoride which coinprises subjecting the concentrates at moderately elevated temperatures to the action of sulfuric acid in aslurry containing approximately solids and approximately 100 pounds of the'acid per ton of dry concentrates in the presence of a sufiicient quantity of an aluminum-bearing compound to provide aluminum in stoichiometric equivalency to the fluorine in the concentrates, maintaining the slurry ata moderately elevated temperature less than 100 C. for a time suificient to effect ,complete decomposition of the calcium fluoride," then separating the liquid from the solids andwashing the latter to thereby remove therefrom in solution the fluorine content of said fluoride.

3. A method of purifying finely divided fluorspar-bearing zinc sulfide comprising-the steps of mixing with a water pulp of, the sulfide about 100 pounds of sulfuric acid and up to about 50 pounds of bauxite per ton of the dry sulfide, maintaining the mixture at a moderately elevated tempera-' ture u'ntil conversion of the fluorinezcontent of the fluospar to relatively highly. soluble compounds and then separating said soluble compounds in solution from the 'undissolved'sulfide. 4. The method of removing fluorine from zinc sulfide concentrates containing calcium fluoride in quantity not substantially in excess of 2.5% which comprises digesting, the concentrates in water with sulfuric-acid in substantial excess and an aluminum-bearin compound relatively small excess of the respective stoichiometric equivalents of the calcium'fluoride present in the p 1 concentrates while supplying heat to the mass'to maintain its temperature from about 50 C. to about C. for a time of the order of one to four hours to thereby fully decompose the fluoride and then filtering and washing the concentrates to remove the fluorine therefrom in solution.

5. In a method of purifying fluorspar-contaminated zinc concentrates, the steps of mixing with a water pulp of the concentrates containing about 60% solids, about pounds of concentrated sulfuriclacid per ton of dry solids in the pulp and a suificient quantity of an aluminum-bearing compound to provide an appreciable excess of aluminum over the stoichiometrical equivalent of the fluorine present in the pulp and agitating I the mixture at an elevated temperature not exceeding about 90 C.

6. In a method of purifying fluorspar-contaminated zinc concentrates, the steps of mixingwith a water pulp of the concentrates containing about 60% solids, about 100 pounds of 'concen-,

,trated sulfuric acid and up to about 50 pounds of bauxite per ton of dry solids in the pulp and agitating the mixture while maintaining its tem- 50C. to about 90 C. CARL O. ANDERSON. RUDOLPH J. STENGL. v FREDERICK C. ABBOTT.

perature from about 

